1. Field of the Invention
This invention relates to electro-optic modulators in which an optical signal is modulated by an electrical signal, and more particularly to modulators in which segmented modulating electrodes are distributed along an optical network.
2. Description of the Related Art
A trade off must normally be made between bandwidth and power when designing an electro-optic modulator that has an extended optical transmission network, such as a Mach-Zehnder interferometer. Longer modulators result in a narrower bandwidth, while shorter modulators have a greater power consumption.
At frequencies above 2 GHz, the electric signal propagates along with the optical signal down the length of the modulator. This allows the electric signal to interact with the optical signal over the length of the modulator. The reason for the reduction in bandwidth in these traveling wave modulators as the modulator length increases is that the propagation speed of the electrical radio frequency (RF) modulating signal is less than the optical propagation speed through the modulator. The modulator operation begins to degrade when the two signals go more than 180.degree. out of phase with each other before the end of the modulator; the amount of phase differential is proportional to the modulator length. Since RF signals have shorter wavelengths at higher frequencies, and therefore go out of phase with the optical signal more rapidly at such frequencies, longer modulators encounter a high frequency bandwidth limitation.
The additional power requirement of a shorter modulator is related to the depth of modulation, which is the ratio of the difference in optical output intensity between the positive and negative RF peaks (the RF signal normally rides on a DC bias), divided by the maximum optical output intensity with zero modulation. The depth of modulation is directly proportional to the modulation drive voltage times the length of the modulator. It decreases as the modulator length is reduced, since the interaction length of the RF and optical signals is reduced. Accordingly, a stronger modulation drive (i.e., greater RF voltage) is required to maintain the same depth of modulation for a shorter modulator. The power required to produce a given depth of modulation varies inversely with the square of the modulator length.
An extended modulator length with a consequent reduction in the RF drive power is disclosed in Schaffner et al. U.S. Pat. No. 5,005,932, issued Apr. 9, 1991 and assigned to Hughes Aircraft Company, the assignee of the present invention. In this approach a periodic electrode structure is used to match the velocity of the optical signal with that of the dominant RF space harmonic. The RF optical signals are perfectly coherent at one frequency so that the modulator response is high at the design frequency, but away from this frequency the modulator response decreases. The frequency response is thus quite limited.
Another time delay modulator is described in Bridges U.S. Pat. No. 5,076,655, issued Dec. 31, 1991 and assigned to Hughes Aircraft Company, and in Bridges et al., "Wave-Coupled LiNbO.sub.3 Electroptic Modulator For Microwave and Millimeter-Wave Modulation", IEEE Photonics Technology Letters, Vol. 3, No. 2, February 1991, pages 133-135. The electrodes of an electro-optic modulator are divided into short sections, with each section connected to its own antenna on the surface of a common substrate. The antennas are then illuminated from the underside of the substrate by a plane wave that is transmitted through the substrate at an angle to the modulator surface; the angle is chosen so that the phase velocity of the incident modulating wave in the direction of the optical waveguide is equal to that of the optical wave. Each antenna thus receives the proper phase to match the phase velocity of the optical wave. Phase errors are kept small by making the sections short, while power requirements are limited because of the extended length of the overall modulator. A simple phase modulator was disclosed, but the same approach was said to work if applied to one arm of a Mach-Zehnder amplitude modulator.
While the modulator described in the Bridges et al. article achieves a desirable combination of relatively broadband operation coupled with a relatively low power consumption, it requires the presence of an RF transmission antenna on the underside of a LiNbO.sub.3 substrate. The substrate material has a large RF index of refraction, and the receiver antennas therefore collect the RF signal more efficiently than would be the case if the signals were transmitted through the air. The result is a bulky three-dimensional structure that requires a large quantity of relatively expensive LiNbO.sub.3. In addition, the angle of the transmission antenna to the LiNbO.sub.3 substrate is critical and would be difficult to maintain in a system subject to environmental conditions other than those found in the laboratory.